In transverse cutting machines in which a web of a material, such as paper or cardboard, is subdivided into individual sheets transversely to the web, the sheets are generally fed at very high speeds by a belt-type conveyor to a stacking location at which the sheets are stacked on one another.
Because of the high speed of the sheets fed to the stacking location, it is essential to brake the sheets before they are deposited on the stack in order to allow reliable stacking and prevent a rapidly moving sheet from interfering with the uniformity and continuity of the stacking process.
DE-B 20 000 78 describes a device for this purpose in which the sheets are guided over a suction chamber or box with a fixed perforated surface through which a suction is drawn, the suction box being switched cyclically to draw the rear edge or portion of the sheet against the surface of the suction box and thereby brake the sheet. The sheet is then engaged by belts which run at the sheet deposition speed to the stacking site. The sheet deposition speed, of course, is the speed at which the sheet is deposited on the stack.
Since the subsequent sheet is initially unbraked, its leading edge can pass over the rear edge of the braked sheet so that the sheets overlap and are further displaced in a shingled or overlapping stream.
Since the suction force is only effective on the lower-most sheets, with a multilayer operation in which, for example, eight webs are guided one above the other to the cutter for simultaneously cutting into sheets, in effect, packets of eight sheets must be fed to the stacking location, the entire sheet packet must be braked and this cannot be effectively done utilizing a suction box in the manner described.
For the purpose of handling such packets, belts inclined to the feed plane must be used and the leading edges of the sheet packet can abut the inclined belt or belts.
This type of brake device has been found to be structurally expensive and cannot be used effectively with sensitive papers, i.e. papers which are sensitive to crinkling or marking, since the relative speeds of the elements give rise to undesirable marking of the sheets. At high operating speeds, moreover, the sheets can shift.
To avoid these drawbacks, the International application PCT/EP 90/02143 provides for the braking of the sheets, synchronously operating clamping elements with clamping zones which engage the rear edges of the sheets at least in part in a clamping operation as these clamping elements are circulated.
At the clamping zones where these elements engage the sheets, they are driven at the feed or inlet speed of the sheet. Beyond the clamping zone, however, they are braked to the discharge speed at which the sheets are delivered to the stack. At the lower, discharge, speed, the sheet is frictionally held and the clamping of the sheet is released. The clamping zone is then accelerated to the inlet speed of the sheet before its clamping element engages a successive sheet.
According to an embodiment of this system, to both sides of the transport plane of the sheets, endless belts are provided with nonuniform drives with at least the belts on one side being provided along the exterior with at least one clamping body whose surface defines the circulating clamping zone. The inner stretches of the belt run parallel to or at an acute angle to the transport plane, the inclination being in the transport direction. This device provides reduced strain on the sheets and higher operational reliability.
However, with this system as well as with other prior art systems there are limits to the operating speed since the rear edges of the sheets must be clampingly engaged by an element from above and, cyclically following the deceleration, must be moved again to an open position and brought to the inlet speed. The braking process must be complete before overlapping by the next sheet begins since, for such overlapping, it is desirable to move the clamping body out of the feed plane for the next sheet. Toward the end of the deceleration phase there must be a residual gap before the subsequent sheet is provided to avoid an engagement of the leading edge of that sheet by providing time before its arrival for the clamping body to reach its upper most position.
The time interval which is allowed for this movement of the clamping body and thus which is available between the leading edge of the next sheet and the previous sheet can be increased by separating the sheets even before their braking by relatively large gaps and transporting them on the conveyor with such gaps.
For given basic speeds, an increase in the gaps can be achieved only by accelerating the sheets to a greater inlet speed.
Apart from the fact that the requisite higher acceleration levels prior to braking requires higher braking powers to achieve the relative low outlet speed of the sheet, there is an increasing loading of the sheet which puts limits on the suitability of this structure.